I rode my Dahon Speed D7 (stock tires) through slush and some ice recently here in Brampton when the roads hadn't been cleaned yet....it required my full attention as I had to go as slow as possible and my rear was sliding all over the place. I had to walk it on the sidewalk uphill for a few minutes at one point. What I realized that day was one big advantage of a folder. I can easily raise the handlebars and lower the seat post so that I'm sitting very upright and my feet easily touch the ground. Folders have a wider range of adjustments then non folders.

I also realized another thing. That slush STICKS to the bike all over the place. I had bounced my bike on the pavement to get rid of most of the slush. Brought it into Starbucks and within 10 minutes there was a huge puddle. Not sure how many nooks and crannies the slush was in. Next morning the chain had started to have some surface rust....not going to be riding in slush ever again unless I can help it.

Roads here are generally good so it's not worth getting winter tires for me especially this years short winter. I just choose cleaner paths and pay more attention. Plus it's not like I commute to a real job where I will be penalized for staying home a few days out of the month.

Either way, +1 on folders for winter. Just bring some water along to rinse off the bike before throwing it in the car or wherever you put it. Maybe even an old towel or something.